Re: [LKP] Re: [mm/hugetlb] c77c0a8ac4: will-it-scale.per_process_ops 15.9% improvement

[Waiman Long]

On 1/17/20 1:56 AM, Feng Tang wrote:
> Hi Waiman and Michal,
>
> On Tue, Jan 14, 2020 at 09:57:14AM -0500, Waiman Long wrote:
>> On 1/14/20 4:12 AM, Michal Hocko wrote:
>>> On Tue 14-01-20 16:56:37, kernel test robot wrote:
>>>> Greeting,
>>>>
>>>> FYI, we noticed a 15.9% improvement of will-it-scale.per_process_ops due to commit:
>>>>
>>>>
>>>> commit: c77c0a8ac4c522638a8242fcb9de9496e3cdbb2d ("mm/hugetlb: defer freeing of huge pages if in non-task context")
>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
>>> This is more than surprising because the patch has only changed the
>>> behavior for hugetlb pages freed from the (soft)interrupt context and
>>> that should be a very rare event. Does the test really generate a lot of
>>> those?
>>>
>> Yes, I have the same question. I was not expecting to see any
>> performance impact.
> We have the same question and did some further check.
>
> This is the "pagefault3" test case of will-it-scale, and is  
> mmap/get_page/munmap test. The source code is: 
> https://github.com/antonblanchard/will-it-scale/blob/master/tests/page_fault3.c 
>
> And its running on LKP does NOT involve any hugetlb actions, as
> could be checking HugePages_* in /proc/meminfo.
>
> We also did another check, reverted c77c0a8ac4c5 and simply added
> some printk inside free_huge_page(), which can also bring 15%
> improvement.
>
> So one possible reason could be the commit changes the cache
> alignment of other kernel codes in final bzImage, which happens
> to hugely affect this test case.
>
> Thanks,
> Feng
>
That sounds reasonable to me. Thanks for the investigation.

Cheers,
Longman